Method of heat treating elongated steel articles



Aug. 11, 1964 J. D. DITSON 3,144,365

METHOD OF HEAT TREATING ELONGATED STEEL ARTICLES Filed July 10, 1963 2 Sheets-Sheet 1 F/GJ HARD/VESS TEMPERATURE HARDENING ZONE UNA FFECTE D ZONE IN VENTOR.

.1. 0. a/rso/v 9w wfmmf ATTO RNEY LE/VETH United States Patent 3,144,365 METHOD OF HEAT TREATING ELONGATED STEEL ARTICLES J. D. Ditsou, West Portal, N.J., assignor t0 Ingersoll-Rand Company, New York, N .Y., a corporation of New Jersey Filed July '10, 1963, Ser. No. 293,986 4 Claims. (Cl. 148145) This invention relates to a method for heat treating elongated steel articles.

Frequently, it is necessary to heat treat a portion of an elongated steel article which has been previously heat treated. The use of this procedure may be caused by the need to give different portions of the piece different values of hardness; or, alternately, the piece may be too large for conveniently heat treating it simultaneously along its entire length. For example, the rock drill manufacturing industry often finds the need to heat treat different portions of a piece separately during the manufacture of rock drilling steels or rods.

The use of two or more heat treatments on different portions of a steel piece inherently creates a metallurgical notch between the two heat treated portions. A metallurgical notch is a soft or fully annealed zone joining the two portions of heat treated steel and is caused by the temperature gradient which inherently exists between the two portions of steel during the later heat treatment. This metallurgical notch zone is a weak section of the heat treated piece because it extends transversely across the entire cross-sectional area of the piece.

The principal object of this invention is to solve the metallurgical notch problem and to provide a method for heat treating portions of an elongated steel article separately without creating a weak metallurgical notch zone which makes the article more likely to break or give in the area of the metallurgical notch.

The invention is illustrated in the drawings wherein:

FIG. 1 is an elevational view of a portion of a drill steel showing two sections which have been separately heat treated;

FIG. 2 is a curve illustrating the hardness of the drill steel of FIG. 1 at various portions of its length;

FIG. 3 is a curve illustrating the temperature gradient existing in one of the sections of drill steel during its heat treatment;

FIG. 4 is a diagrammatic elevational view illustrating an embodiment of the method of this invention;

FIG. 5 is an axial section of the drill steel of FIG. 4; and

FIG. 6 is an enlarged section taken on the line 66 of FIG. 5.

A steel rock drill rod 1 is shown in FIG. 1 which is subject to two separate heat treatments for hardening purposes. The drill rod 1 is divided into two sections, section A and section B. The first heat treatment provides section B with a selected value of hardness, as shown in the curve in FIG. 2 by the curve portion 2. The second heat treatment is carried out on section A to provide it with a lesser value of hardness, as shown in FIG. 2 by the curve portion 3.

During the second heat treatment of section A, an inherent temperature gradient exists between sections A and B, due to the differential temperatures of these two sections. This temperature gradient is shown in FIG. 3 by a curve having a vertical ordinate designating temperature and a horizontal ordinate designating length of the drill rod 1.

Looking at FIG. 3, the temperature of section A is in the hardening region over most of its length. However,

the portion of the drill rod 1 joining sections A and B, extending generally along the dashed line 4, has a lower temperature located in the annealing region of the steel rod. As a result, this portion or region 4 of the rod 1 will be annealed and much softer and weaker than the remainder of the drill rod 1. The annealed portion of the drill rod 1 is known in the industry as a metallurgical notch. It obtains its name from the dip 5 in the hardness curve shown in FIG. 2.

The metallurgical notch is an inherent result of using a second heat treatment on the drill rod 1. It cannot be avoided. This invention recognizes this fact and does not attempt to prevent it but provides a way that the undesirable characteristics of the metallurgical notch can be eliminated. In other words, this invention strengthens the metallurgical notch so that it is no longer the weakest part of the drill rod 1.

This is carried out by subjecting the surface of the metallurgical notch area with a heat treatment of the type which will provide the drill rod 1 with a hardened cylindrical case 10 surrounding and extending along the metallurgical notch area of the drill rod. The hardened case 10 is illustrated in FIGS. 5 and 6.

The case 10 on the drill rod 1 can be formed by subjecting it to electrical induction heating currents. This can be provided by passing it relatively through an electrical induction coil 11 as shown in FIG. 4. Either the coil 11 can be moved along the drill rod 1 or the drill rod moved axially through the coil 11. The case 10 is formed by rapidly heating the surface of the drill rod 1 to hardening temperature and then cooling it rapidly so that the major portion of the rod underlying the case 10 never reaches an annealing temperature. Of course, a thin portion of the rod 1 immediately under the case 10 will be annealed but this portion is thin and parallel to the rod axis so that it does not have much effect on the strength of the rod 1.

The hardened case 10 extends over the metallurgical notch area and thus prevents the weak metallurgical notc from extending perpendicularly across the entire area of the drill rod 1. Any failure of the drill rod 1 in the metallurgical notch area must extend axially along the case 1 before reaching the surface of the drill rod 1. Hence, the case 10 acts to materially strengthen the drill rod 1 along the metallurgical notch area.

In addition, the case 10 can be formed by flame hardening which is carried out by applying high temperature torch flames to the surface of the rod 1 for a short time and thereafter rapidly cooling the heated surface.

Although a preferred embodiment of the invention is illustrated and described in detail, it will be understood that the invention is not limited simply to this embodiment but contemplates other embodiments and variations which utilize the concepts and teachings of this invention.

Having described my invention, I claim:

1. A method of heat treating an elongated piece of steel to provide it with two adjoining elongated sections which are hardened by separate heat treatments, said method comprising:

(a) heating and cooling the steel piece to harden at least a part of the steel piece including two adjoining elongated sections which are segments of the length of the steel piece;

(b) heating and cooling one of said elongated sections of the steel piece to harden that section of the steel piece while unavoidably creating an annealed region in the piece between the later hardened section and the earlier hardened section of the piece; and

(c) subjecting the surface of said annealed region to hardened case forming temperatures and rapidly 3 4 cooling the surface to create a hardened case which annealed region from being heated to annealing temperasurrounds and strengthens said annealed region. tures. 2. The method of claim 1 wherein said annealed region is subjected to electrical induction heating currents. Refer es Ci i h file f hi patent 3. The method of claim 2 wherein said piece of steel 5 is moved relatively axially through an electrical induc- UNITED STATES PATENTS tion coil to create said hardened case in said annealed 2,145,864 Denneen et al. Feb. 7, 1939 region. 2,327,129 Ronan Aug. 17, 1943 4. The method of claim 1 wherein the surface of said 2,845,377 Seulen et al July 29, 1958 annealed region is subjected to a high temperature flame 10 2,882,191 Van Swaal Apr. 14, 1959 and thereafter cooled with sufiicient rapidity to prevent 2,888,373 Cherrie et al. May 26, 1959 the major portion of the piece under the surface of the 

1. A METHOD OF HEAT TREATING AN ELONGATED PIECE OF STEEL TO PROVIDE IT WITH TWO ADJOINING ELONGATED SECTIONS WHICH ARE HARDENED BY SEPARATE HEAT TREATMENTS, SAID METHOD COMPRISING: (A) HEATING AND COOLING THE STEEL PIECE TO HARDEN AT LEAST A PART OF THE STEEL PIECE INCLUDING TWO ADJOINING ELONGATED SECTIONS WHICH ARE SEGMENTS OF THE LENGTH OF THE STEEL PIECE; (B) HEATING AND COOLING ONE OF SAID ELONGATED SECTIONS OF THE STEEL PIECE TO HARDEN THAT SECTION OF THE STEEL PIECE WHILE UNAVOIDABLY CREATING AN ANNEALED REGION IN THE PIECE BETWEEN THE LATER HARDENED SECTION AND THE EARLIER HARDENED SECTION OF THE PIECE; AND (C) SUBJECTING THE SURFACE OF SAID ANNEALED REGION TO HARDENED CASE FORMING TEMPERATURES AND RAPIDLY COOLING THE SURFACE TO CREATE A HARDENED CASE WHICH SURROUNDS SAND STRENGTHENS SAID ANNEALED REGION. 